The LTE (Long Term Evolution) has become widespread as a common specification for radio communication systems in place of third-generation mobile communications systems (3G: 3rd Generation). The LTE is a telecommunications specification that supports only packet communications, and voices are converted into packets according to VoIP (Voice over Internet Protocol) and the converted packets are then transmitted and received. Particularly, the VoIP of the LTE specification is called VoLTE (Voice over LTE).
Regarding a mobile communications system, a home base station is installed and made to relay communication between a terminal device and a core network in order to secure communication quality within a building. Conventionally, a fixed line has been used for communication from the home base station to the core network (for example, see PTL 1).
When the fixed line is used as a backhaul like in a technique described in PTL 1, there are problems such as time required to install the fixed line and running costs for the fixed line. Accordingly, the use of radio communication for the backhaul communication is proposed as described in PTL 2.